Brittany Murphy's Mom: My Daughter Didn't Do Drugs

In their first interview since her death in December, the mother and widower of actress Brittany Murphy tell PEOPLE that they believe she died because of a congenital heart murmur.

"She had a little bit of a cold," says Murphy's mother Sharon of the days leading up to the actress's death. "You could never in a billion years imagine this happening."

Despite rumors that have swirled since her passing, both Sharon Murphy and Brittany's husband, Simon Monjack, say the 32-year-old did not suffer from mental illness, use drugs or have an eating disorder. "Do you not think anybody would have picked up on this anorexic, mentally deranged person?" Monjack tells PEOPLE. "It's ludicrous."

The pair have spent the last month sequestered in Murphy's Hollywood home. PEOPLE was the first media outlet allowed into the residence.

Both say Murphy's heart condition was diagnosed in her teens and that she had been given appropriate medical care. "We had it checked out every which way you could imagine," says Monjack.

Murphy's mother says her daughter's naturally "bubbly" personality may have sparked rumors of drug use. "She never even drank. Maybe a glass of champagne at New Years," says Sharon. " But everyone used to say she was wasted, she was this, she was that. It was hard for anyone to imagine that somebody was so high on life."

For an in-depth interview with Sharon Murphy and Simon Monjack and an exclusive look inside the actress's Hollywood home, check out next week's PEOPLE, on sale Jan. 22.